This invention relates to a multicolor recording apparatus which employs ink donor sheets for recording information on ordinary paper. More particularly, it relates to a multicolor recording apparatus capable of restricting the color displacement in the picture image only locally.
It is known that it is possible to record information on ordinary paper by heat transfer if the paper is brought into contact with a hot-melt ink layer formed on one surface of an ink donor sheet, and the ink layer is selectively heated by a thermal head in accordance with pictorial information so that the ink may be melted and transferred to the recording paper.
It is also known that multicolor recording can be carried out in a thermal transfer recording system by preparing a plurality of types of ink donor sheets respectively coated with solid ink of different hues, and carrying out successive thermal transfer recording operations with these ink donor sheets onto a single sheet of recording paper.
In a multicolor recording apparatus using such a recording system, a plurality of recording stations for different colors are provided on a conveying passage for the recording sheet at predetermined intervals with each other. Upon recording at one station, the recording sheets are sent along the conveying passage to the next station, and picture signals of hues corresponding to each recording station are supplied to each of the respective recording stations to conduct multicolor recording. In this case, for the second or succeeding recording station disposed on the conveying passage, picture signals are delayed by a time period previously calculated depending on the running speed of the recording sheet and the distance from the previous recording station, so that the positions for the picture images recorded in each of the recording stations are aligned.
FIG. 1 shows a conventional two-color recording apparatus employing two different kinds of ink donor sheets. The apparatus comprises two recording stations, i.e., a p-color recording station 1 and a g-color recording station 2. The letters p and g designate two different colors, such as red and black, respectively.
The p-color recording station 1 comprises a p-colored sheet supply roll 11, a p-color thermal head 13 contacting the base or rear surface of a p-colored ink donor sheet 12 supplied from the roll 11, a p-color back roll 14 positioned opposite to the thermal head 13 for pressing ordinary paper 3 and the p-color ink donor sheet 12 into contact with the thermal head 13, and a p-color take-up roll 15 which collects the p-colored ink donor sheet 12 after the ink has been transferred therefrom onto the paper 3. Likewise, the g-color recording station 2 comprises a g-colored sheet supply roll 21 on which a g-colored ink donor sheet 22 is wound, a g-color thermal head 23, a g-color back roll 24, and a g-color take-up roll 25.
When the ordinary paper 3 is transported in the direction of an arrow A, and the p- and g-colored ink donor sheets 12 and 22 in the direction of an arrow B, p-colored pictorial information is recorded on the paper in the p-color recording station 1, and g-colored information is then recorded in the g-color recording station 2.
Each of the thermal heads 13 and 23 has a certain thickness when viewed in the direction of travel of the paper 3. The ink donor sheets passing about the thermal heads must be kept from contacting each other. Thus, it is necessary to provide a distance of at least about 50 mm between the heating resistors in the two thermal heads 13 and 23. This results in a requirement for a memory having a very large capacity for delaying the image signal to be fed to the g-color thermal head.
If, for example, paper of the A4 size conforming to the Japanese Industrial Standard (which has a width of 210 mm) is used as ordinary paper, the heating resistors in the thermal heads have an image element density of 8 dots/mm, 7.7 scanning lines per mm, and a distance of 50 mm from each other, the above-described two-color recording apparatus requires a memory of 646,800 bits (210.times.8.times.50.times.7.7). A memory having three times as large a capacity is required for three-color recording, since the signals must again be delayed by twice as much between the first and third stations. This has been one of the factors which make this type of recording apparatus expensive.
Moreover, as it is impossible to position the thermal heads sufficiently close to each other, paper is very likely to slacken between the heads, resulting in a substantial possibility of colored images being recorded in the wrong positions at subsequent stations, thereby detracting from the image quality.